239. Sliding Window Maximum

Using monotonic queue to have the biggest element of the sliding window at the start of the queue
We are using deque because it allows us to remove elements from rear (when the new element is bigger than the smallest element in the window)
We have the first while to make sure the big number which is out of the window is removed
We have the second while to make sure we remove all the elements smaller than the new element in the queue before adding the new element
We only start adding the index to the res array when the window covers the minimum size

Space - O(n), the res & deque take n heap spaces in the worst case
Time - O(n), we need to process each element once, and each element will be offered once & pop out at most once